Headphone socket wiring / schematic

[mcandmar]

Hi All,

I'm about to order a S.E.X. kit and have a query re the impedance switch kit.  Instead of using the switch kit is there any reason why i cant install two headphone sockets into the amp and wire one to the 4ohm tap, and one to say the 32ohm tap?

Also is it possible to download a schematic for the amp, i am interested to see the headphone output section re the above query.

Thank you,

Mark

[Paul Birkeland]

Instead of using the switch kit is there any reason why i cant install two headphone sockets into the amp and wire one to the 4ohm tap, and one to say the 32ohm tap?

This would be possible if there was strictly a 4 Ohm and a 32 Ohm tap, but the output transformers have multiple secondaries that are series/parallel wired to achieve the different impedances. 

For some graphical representations of this, you can check out the hookup diagrams for these transformers. 

[mcandmar]

Ahh i see. Thanks for the reply..

[Paul Birkeland]

No problem, as you look at the photos of the impedance switch kit, it will help explain why there are so many darn switches in there!

[mcandmar]

Thanks Paul, i have ordered the switch kit with the amp so no doubt i will back with more questions when i get to the assembly stage.

Somewhat related, i'm going to order a TRS and XLR Neutrik socket from Mouser for the build and would like to add a few other bits and bobs into the order.  Is there anywhere i can get a list of resistors used, values and wattage?

Or is there any way i get a copy of the assembly manual in advance once my order is confirmed?

Thanks,

Mark

[Paul Birkeland]

Sure, you can give us a call and we can shoot you a download link.

[mcandmar]

Right, i have a standard TRS and XLR connector installed, TRS is wired to + and Gnd on each impedance board, XLR is wired to + and - on each impedance board.

I've been studying the PCB trying to work out what the Balanced/Unbalanced switch is doing and its just making my head hurt.  All i can figure out is it adds the two resistors into the path?

What i'm really trying to understand is how the switch will effect the TRS or XLR socket in either position.   For example am i right in saying the switch will have no any effect on the standard TRS + and Gnd wiring, it only switches the - rail used on the XLR socket from balanced to unbalanced?

Thanks,

Mark

[Paul Joppa]

Here is the wiring diagram, if that helps.

There are two windings, both tapped. SW1 chooses which tap is used; SW2 chooses whether they are in series or parallel. If they are in series then the centertap is available for balanced ground; but if they are in parallel there is no center tap so one is created with two resistors.

In theory, balanced operation does not need a ground reference, but in practice I feel all headphone wiring should be referenced to the safety (chassis) ground.

It took me along time to come up with this simple diagram, and it made my head hurt, too!  :^)

Incidentally, it is important that both windings be used equally; otherwise the treble response suffers.

[mcandmar]

Excellent thanks Paul, i'll have to get my thinking cap on to digest that...

[mcandmar]

I think i am slowly getting my head around this, Paul can you please confirm if the following are true or false,

-With 4 & 8 ohms the windings are in Parallel, 16 & 32 ohms the windings are in series.
-When wired in parallel the center tap is devised via resistors from each winding, and when wired in Series the center tap is the link between both windings.
-When in unbalanced mode the output transformer negative rail is tied to chassis ground so GND and NEG become the same, and the center tap devised above is left floating.
-When in balanced mode the center tap devised above is connected to ground, and the transformer negative rail becomes the NEG tap for balanced operation.

-With an XLR jack, if the switch is set to unbalanced mode its going to function no different from a regular unbalanced TRS jack as the negative points become grounds.
-With a TRS jack, if the switch is set to balanced mode its only going to be connected to half of the output transformer as the ground rail is connected to the output transformer center tap, not the negative rail?

Now i just need to figure out what the optimum configuration is for headphone only use in balanced and unbalanced modes.  i.e Balanced with 300ohm load (XLR), and unbalanced with 32ohm loads (TRS).

-I've read the lower the ohm setting the lower the noise floor.
-I assume 16 & 32 ohm mode is preferable for balanced operation due to the real center tap vs the resistor devised center?
-Manual recommends 32ohm setting for headphone only use, would love to know the reasoning for that?

Thanks again for your patience, i have almost finished this build and will promise to stop bugging you guys

[Doc B.]

Unless you can't get the music loud enough choose the 8 ohm tap. It seems to work well with just about every headphone. The balanced output sounds the same whether going thru resistors to virtual ground or thru the transformer secondary center tap, so I wouldn't be concerned about that.

Reducing the noise floor as much as possible is a much more audible "tweak". After some time with the revised circuit I would recommend only to use the 32 ohm tap if one actually came across very inefficient high impedance headphones that need the voltage swing available at that tap. Otherwise, with more sensitive headphones you will find that the noise floor can be slightly audible at the 32 ohm tap.

If you have a lean sounding headphone like some of the AKGs the 16 ohm tap might give them a bit more weight then the 8 ohm tap. But again you might start to get to a point where the background noise floor affects the sense of blackness of the background between notes. This is all subtle stuff I am talking about, you just have to smoke it and see.

And of course the idea behind the impedance switching kit is that one can easily determine all of this for one's self and avoid having to rely upon other's opinions.

[mcandmar]

Great info thanks Doc.   I fired it up on the lowest 4ohm setting to start with and plugged in a pair of Grados, seems perfect as with the volume dial at 50% its about as loud as i would want it, but i will experiment a bit.  I am getting a faint hum in the backround i need to sort out but so far it sounds good, really good.  When my MS-Pros plugged in it gave me eargasms, multiple eargasms

[Paul Joppa]

mcandmar, you pretty much have it. Here's the only one that is actually more complicated:

"-With a TRS jack, if the switch is set to balanced mode its only going to be connected to half of the output transformer as the ground rail is connected to the output transformer center tap, not the negative rail?"

* TRS jack, set to balanced, and at 16 or 32 ohms: Then you are actually listening to only one of the secondary windings, making it effectively a 4 or 8 ohm winding. If you had 4 or 8 ohm headphones, you would lose a couple octaves of treble due to the increase of leakage inductance from incorrect usage. With headphones above 16 or 32 ohms, this effect is pretty much gone.

* TRS jack, set to balanced, and at 4 or 8 ohms: Now the windings are paralleled for proper transformer operation, but the resistors are in the circuit. One resistor is across the headphone jack, and the other one is in series with the winding voltage. Effectively you get half the voltage, with a source impedance of 30 ohms. This reduced output could actually be useful for high-sensitivity low-impedance TRS phones making them more compatible with high-impedance balanced phones. But in some cases the additional source impedance will reduce the damping of the headphone and make a more wooly or bloated sound.

Now my head hurts.

[mcandmar]

Is there any technical reason why you shouldn't switch the impedance board while the amp is running?

The manual for the impedance boards states "UNPLUG AMP BEFORE SWITCHING!", however is that just a safety warning to discourage people from poking their fingers inside a live amp, or is it ok to lift the amp and switch them with something non conductive?

[Paul Birkeland]

Is there any technical reason why you shouldn't switch the impedance board while the amp is running?

The manual for the impedance boards states "UNPLUG AMP BEFORE SWITCHING!", however is that just a safety warning to discourage people from poking their fingers inside a live amp, or is it ok to lift the amp and switch them with something non conductive?

Yeah, there's a good chance that you'll zap yourself while you reach under there.  On top of this, the zap will cause you to drop the amplifier, and it may zap you again on its way down.  (Don't ask how I know)